The invention relates generally to the field of fiber optics, and in particular to a method and apparatus for coupling light emitted from a multi-mode laser diode array to a multi-mode optical fiber.
Systems incorporating a laser light source optically coupled to an optical fiber are well known and find an ever-increasing variety of applications. Such systems include fiber optic communications systems, fiber optic medical instruments, welding equipment, laser copiers, laser printers and facsimile machines.
FIG. 1 is a cross-sectional view of a conventional optical fiber having a core 10 and cladding 20 and shows that an angle .theta..sub.c defines a maximum launch angle, .phi..sub.c, at which guided light rays 30 may be injected into the optical fiber. This launch angle is usually given in terms of numerical aperture (NA) which is commonly defined by the following equation: ##EQU1## where n.sub.1 is the index of refraction of the core 10 and n.sub.2 is the index of refraction of the cladding 20.
Since energy launched at angles greater than .phi..sub.c is rapidly attenuated, coupling efficiency of a laser diode to a fiber depends upon NA.
High-power laser diodes are usually edge emitting multi-mode lasers. These laser diodes consist of either a single, broad area emitting aperture commonly 1 micron by 200 microns or they are arranged as an array of smaller sized apertures. A divergence angle of the light emitted by such a laser diode array through an array of smaller sized apertures is typically 30 degrees by 10 degrees where the smaller divergence (10 degrees) is the divergence measured along the edge (the longitudinal) direction.
Optical systems are frequently described by an optical invariant, or Lagrange, which is constant for a given optical system. A numerical value for the Lagrange may be calculated in any one of a number of ways known in the art and the Lagrange may then be used to arrive at a value for other quantities of optical significance. The Lagrange in each direction for a laser diode array is defined as half the size of the emitting aperture times half the divergence angle (in radians). In the typical case, the Lagrange invariant in one direction (the "array" direction or long dimension of the emitting aperture) is denoted by L.sub.a, and defined as: EQU L.sub.a =(200/2) * (5/57.3)=8.5 microns
where 57.3 is the conversion factor for converting degrees to radians. The Lagrange invariant in the other direction, i.e., the "cross array direction" or short dimension of the emitting aperture, L.sub.ca is: EQU L.sub.ca =0.5 * 15/57.3=0.13 microns
Since optical fibers are typically radially symmetric, their Lagrange, L.sub.fiber is the same in any direction and is therefore given by: EQU L.sub.fiber =a * NA
where a is the fiber core radius and NA is the Numerical Aperture as previously described.
The capacity of optical systems to transmit light is called "the optical throughput" or "etendue". The optical throughput is an invariant of the non-truncating, non-absorbing and non-diffusing optical system and is proportional to the Lagrange invariants in two directions, namely L.sub.a * L.sub.ca. When an optical system with a large optical throughput is coupled to an optical system with a lower optical throughput, light loss occurs. Such light loss is avoided when the optical throughput of a receiving system is greater than or equal to the optical throughput of a transmitting system. One way to ensure that the optical throughput of the receiving system is greater than or equal to the optical throughput of the transmitting sywstem, is to have both L.sub.a and L.sub.ca of the receiving system greater than or equal to the L.sub.a and L.sub.ca of the transmitting system.
For light to be efficiently coupled into the fiber therefore, the Lagrange of the fiber has to be larger than the Lagrange of the laser diode array in both directions. Furthermore, it is desirable to inject light from the laser diode into a fiber having the smallest L.sub.fiber possible. In laser printing, for example, where an end of the fiber is imaged onto a media, a lower L.sub.fiber results in either a smaller spot of light at the media or a larger depth of focus. Since L.sub.a &gt;L.sub.ca, it is desirable to minimize L.sub.a so as to permit the coupling of the laser light into a lower L.sub.fiber.
It is desirable therefore to optically couple a multi-mode laser diode array to an optical fiber in a manner which reduces the overall L.sub.a such that the power and radiance of the laser light transmitted through the optical fiber is increased.